11th Chemistry Volume · Part

H C Similar to the other electron displacement effect, mesomeric effect is also classified into positive mesomeric effect (+M or +R) and negative mesomeric effect (-M of -R) based on the nature of the functional group present adjacent to the multiple bond. Positive Mesomeric Effect: Positive resonance effect occurs, when the electrons move away from substituent attached to the conjugated system. It occurs, if the electron releasing substituents are attached to the conjugated system. In such cases, the attached group has a tendency to release electrons through resonance.

These electron releasing groups are usually denoted as +R or +M groups. Examples : -OH, -SH, -OR,-SR, -NH , -O- etc. Negative Mesomeric Effect Negative resonance effect occurs, when the electrons move towards the substituent attached to the conjugated system. It occurs if the electron withdrawing substituents are attached to the conjugated system.

In such cases, the attached group has a tendency to withdraw electrons through resonance. These electron withdrawing groups are usually denoted as -R or -M groups. Examples : NO , >C=O, -COOH,- C ≡ N etc Resonance is useful in explaining certain properties such as acidity of phenol. The phenoxide ion is more stabilised than phenol by resonance effect(+M effect) and hence resonance favours ionisation of phenol to form H + and shows acidity.

The above structures shows that there is a charge separation in the resonance structure of phenol which needs energy, where as there is no such hybrid structures in the case of phenoxide ion. This increased stability accounts for the acidic character of phenol. Hyper conjugation The delocalisation of electrons of σ bond is called as hyper conjugation. It is a special stabilising effect that results due to the interaction of electrons of a σ -bond (usually C-H or C-C ) with the adjacent, empty non-bonding p-orbital or an anti- bonding σ* or π*- orbitals resulting in an extended molecular orbital.

Unlike electromeric effect, hyper conjugation is a permanent effect. It requires an α-CH group or a lone pair on atom like N, O adjacent to a π bond (sp hybrid carbon). It occurs by the overlapping of the σ-bonding orbital or the orbital containing a lone pair with the adjacent π-orbital or p-orbital. Example : In propene, the σ -electrons of C-H bond of methyl group can be delocalised into the π -orbital of doubly bonded carbon as represented below.

·· ·· ·· (I) (II) (IV) (III) No bond resonance structures shown by propene are due to hyperconjugation In the above structure the sigma bond is involved in resonance and breaks in order to supply electrons for delocalisation giving rise to new canonical forms. In the contributing canonical structures: (II), (III) & (IV) of propene, there is no bond between an α-carbon and one of the hydrogen atoms. Hence the hyperconjugation is also known as “no bond resonance” or “Baker-Nathan effect”. The structures (II), (III) & (IV) are polar in nature.

Example : Hyper conjugation effect is also observed when atoms / groups having lone pair of electrons are attached by a single bond, and in conjugation with a π bond . The lone pair of electrons enters into resonance and displaces π electrons resulting in more than one structure H C